gedit 3.32.0 build with meson fails on macOS

It would appear that the meson build script was not properly updated for macOS.

This is the patch I had to apply to get things working again. For this to work, you also need to rename gedit-app-osx.c to gedit-app-osx.m, and gedit-file-chooser-dialog-osx.c to gedit-file-chooser-dialog-osx.m.